Chemical Fingerprinting of HULIS in Particulate Matters Emitted from Residential Coal and Biomass Combustion

Identification of humic-like substances (HULIS) structures and components is still a major challenge owing to their chemical complexity. This study first employed a complementary method with the combination of two-dimensional gas chromatography–time-of-flight mass spectrometry and liquid chromatography–quadrupole-time-of-flight mass spectrometry to address low-polarity and polar components of HULIS in PM_2.5 (particulate matter with an aerodynamic diameter less than 2.5 μm), respectively. The combination method showed a significant correlation in identifying overlapping species and performed well in uncovering the chemical complexity of HULIS. A total of 1246 compound species in HULIS (65.6–81.0% for each sample), approximately 1 order of magnitude more compounds than that reported in previous studies, were addressed in PM_2.5 collected in real-world household biomass and coal combustion. Aromatics were the most abundant compounds (37.4–64.1% in biomass and 34.5–70.0% in coal samples) of the total mass in all HULIS samples according to carbon skeleton determination, while the major components included phenols (2.6–21.1%), ketones (6.0–17.1%), aldehydes (1.1–6.8%), esters (2.9–20.0%), amines/amides (3.2–8.5%), alcohols (3.8–17.0%), and acids (4.7–15.1%). Among the identified HULIS species, 11–36% mass in biomass and 11–41% in coal were chromophores, while another 22–35 and 23–29% mass were chromophore precursors, respectively. The combination method shows promise for uncovering HULIS fingerprinting.